Hi, I have a 87 FC Turbo S4

The engine revs fine at idle, but when you start pushing it around second gear it stumbles around 3k rpm.

I originally suspected grounding was the issue, but then after inspection I find all the ground wire intact.

Then I checked out other thread and they said it had something to do with too much back pressure which cause the engine to open its port early or something (not quite sure what it is.)
They also said that by replacing the stock downpipe with a bigger down pipe and cat back and allow higher air flow the problem will go away.

So I started testing with the pressure sensor.

I unplugged the pressure sensor and test drove it and the problem went away. But I know that the problem is still there, and might be something worse then I thought.

I did some research and found that turbo rx7 will blow their engine if there is too much free flow on a stock turbo without porting the wastegate.

So I turn the car off, put the covers on, and came onto this forum and looking for help.

 

Since you have a TII engine, that means that there are no 5th/6th ports like on the NA engine. A free flowing exhaust will probably not help with your issue. You should start with a compression check to verify the engine is healthy, and take a look at the spark plugs and ignition wires when you do that.

There are plenty of threads describing how to beef up the grounds as well. That is a very common problem, and even though the ground wires look to be in tact they could be corroded enough to where they're not making good contact.

 

I took the pressure senesor off and the stumbling went away, I didn't see any thread that did the same thing I did.

So it's not grounding. I think.

 

With the Boost sensor vacuum hose disconnected and plugged and the sensor still plugged into the electrical plug, look at the back of the sensor plug and measure the voltage on the Brown/Red wire w/key to on and see if it falls within spec as stated in the FSM which is between 2.3 and 2.7 volts.